Boost circuits are commonly used in desktop computers where a high power factor is required. A boost circuit is used when the AC line current is required to be in phase with the AC line voltage. FIG. 1 depicts a conventional boost circuit 10. The boost circuit 10 includes a boost inductor 14, a MOSFET transistor 16, a capacitor 20, and diodes 18 and 12. Diode 12 comprises a rectifier bridge circuit. However, this type of conventional boost circuit is not very efficient when used to operate over a wide range of input AC line voltages.
For instance, at a low line input voltage, there is always a two diode voltage drop in the rectifier bridge circuit. Moreover, the bridge circuit is inserted between filter components and the boost circuit and a large heat sink is required for the bridge. This arrangement causes a lot of ElectroMagnetic Compatibility (EMC) problems. Also, the switching losses in the transistor 16 are very high due to the high switching voltage of the bulk voltage (i.e. +V.sub.b =400V) and the large AC line current. Moreover, the steady state loss in the transistor 16 is extremely high due to the high voltage rating MOSFET required. Furthermore, these losses are responsible for thermal problems in circuits where the application requires high power (e.g. 1000W or more).
Accordingly, what is needed is an improved boost circuit that performs more efficiently than conventional boost circuits. The present invention addresses such a need.